Aromatization of Hydrocarbons by Oxidative Dehydrogenation Catalyzed by the Mixed Addenda Heteropoly Acid H5pmo10v2040

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J. Org. Chem. 1989,54, 4607-4610 4607 Aromatization of Hydrocarbons by Oxidative Dehydrogenation Catalyzed by the Mixed Addenda Heteropoly Acid H5PMo10V2040

Ronny Neumann* Casali Institute of Applied Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel 91904 Manfred Lissel* Facultat fur Chemie, Universitat Bielefeld, 4800 Bielefeld 1, Federal Republic of Germany Received February 28, 1989

The mixed addenda heteropoly acid H6PMoloV2040dissolved in 1,2-dichloroethane with tetraglpe, forming the (tetraglyme)~-H6PMolov20~complex, catalyzes the aromatization of cyclic dienes at moderate temperatures in the presence of molecular oxygen. Dehydrogenations of exocyclic dienes such as limonene show that dehydrogenation is preceded by isomerization to their endocyclic isomers. Aromatization takes place by successive one-electron transfers and proton abstractions from the organic substrate to the heteropoly acid, the latter being reoxidized by dioxygen coupled with the formation of water.

Introduction Table I. Oxidative Dehydrogenation of Hydrocarbonso Heteropoly acids and salts, especially those having a time, yield,* Keggin structure, have recently been increasingly reported substrate product h % as catalysts in both oxidative and acid-catalyzed trans- 9,lO-dihydroanthracene anthracene 16 >98, 89' formations.lp2 Heteropoly compounds as liquid-phase 9,lO-dihydroanthracene anthracene 24 - 1-2d oxidation catalysts have been used in various systems 9,lO-dihydrophenanthrene phenanthrene 5 >98 which may be classified by the type of oxidant and whether 9,lO-dihydrophenanthrene phenanthrene 24 - 1-2d the system has been photoactivated. One group of ap- 1,2-dihydronaphthalene (75%) naphthalene 5 >98 tetrahydronaphthalene naphthalene 24 0 plications involves the use of oxidizing reagents such as l,4-cyclohexadiene benzene 18 >98 hydrogen peroxide, tert-butyl hydroperoxide, or iodoso- 1,3-cyclohexadiene benzene 6 >98 benzene in the oxidation of alcohols, allyl alcohols, alkenes, cyclohexene benzene 24 0 and alkane^.^ The use of molecular oxygen coupled with 4-vinyl-1-cyclohexene ethylbenzene 24 22 photochemical activation represents a second set of reac- 1i m on en e p-cymene 20 >98, 7OC tions as in the dehydrogenation of alcohols, alkenes, or "Reaction conditions: 5 mmol of substrate, 1 mmol of tetra- alkane~.~The third area of research includes reactions glyme, 0.05 mmol of HSPMo10VZ040,10 mL of 1,2-dichloroethane, where the heteropoly compound acts as an electron ac- temperature = 70 "C, Pomsn = 1 atm. *Yields as computed by ceptor in what might be termed electron-transfer oxida- GLC. Isolated yields. dReaction performed at 1 atm of nitrogen. tions. In these transformations, oxidation of the substrate gas-phase dehydrogenation of aldehydes and carboxylic is achieved by successive one-electron transfers to the acids to form carbon-carbon double bonds a to the func- heteropoly anion. The reduced heteropoly compound is tional group5 or oxidation of sulfides to sulfoxides and substrate + HPC,, - product + HPCIed sulfones.6 In addition, substrates may be of an inorganic nature such as in the oxidation of bromide anions to molecular bromine.' The substrate may also be a primary reoxidized by oxygen coupled with the formation of water catalyst such as Pd(0) (product is Pd(I1)) in a Wacker type to complete the catalytic cycle. Substrates in the context oxidation reaction! Generally, the most effective catalysts of the above reaction may be organic molecules as in the for this electron-transfer type of oxidation are the mixed addenda heteropolyanions of the formula H3+nPMo12-nVn040,PMoV-n, where n = 2-6. (1) (a) Pope, M. T. Heteropoly and Isopoly Oxymetallates;Springer In this paper we describe the use of H5PMoIoV20, Verlag: New York, 1983. (b) Day, V. W.; Klemperer, W. G. Science 1985, 228,533. (c) Pope, M. T. In Comprehensive Coordination Chemistry; dissolved in an apolar organic phase to catalyze the aro- Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press, matization of cyclic dienes by oxidative dehyrogenation New York, 1987; Vol. 3, Chapter 38. in the presence of dioxygen. In examples where the two (2) (a) Kozhevnikov, I. V.; Matveev, K. I. Russ. Chem. Reu. 1982,51, 1075. (b) Kozhevnikov, I. V.; Matveev, K. 1. Appl. Cat. 1983,5,135. (c) double bonds are not endocyclic, e.g. limonene, aromati- Kozhevnikov, I. V. Russ. Chem. Reo. 1987,56,811. (d) Misono, M. Cat. Reu. Sci. Eng. 1987, 29, 269. R (3) (a) Matoba, Y.; Ishii, Y.; Ogawa, M. Synth. Commun. 1984,14,865. I B (b) Yamawaki, K.; Yoshida, T.; Nishihara, H.; Ishii, Y.; Ogawa, M. Synth. Commun. 1986,16,537. (c) Hill, C. L.; Brown, R. B. J. Am. Chem. SOC. 1986,108,536. (d) Faraj, M.; Hill, C. L. J. Chem. SOC.,Chem. Commun. 1987,1487. (e) Furukawa, H.; Nakamura, T.; Inangaki, H.; Nishikawa, E.; Imai, C.; Misono, M. Chem. Lett. 1988, 877. (0 Schwegler, M.; Floor, I M.; van Bekkum, H. Tetrahedron Lett. 1988, 29, 823. (9) Ishii, Y.; R R Yamawaki, K.; Ura, t.; Yamada, H.; Yoshida, T.; Ogawa, M. J. Org. Chem. 1988,53,3587. (h) Faraj, M.; Lin, C.-H.; Hill, C. L. New J. Chem. 1988, (5) (a) Misono, M. In Proceedings of the Climax 4th International 12, 745. Conference on the Chemistry and Usage of Molybdenum; Barry, H. F., (4) (a) Papaconstantinou, E. J. Chem. SOC.,Chem. Commun.1982,12. Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, 1982; p (b) Darwent, J. R. J. Chem. SOC.,Chem. Commun. 1982, 798. (c) Hill, 289. (b) Konishi, Y.; Sakata, K.; Misono, M.; Yoneda, Y. J. Catal. 1982, C. L.; Bouchard, D. A. J. Am. Chem. SOC.1985,107,5148. (d) Renneke, 77, 169. (c) Otake, M.; Onoda, T. Catalyst 1976, 18, 169. R. F.; Hill, C. L. J. Am. Chem. SOC.1986, 108, 3528. (e) Nomiya, K.; (6) Kozhevnikov, I. V.; Simagina, V. I.; Varnakova, G. V.; Matveev, K. Sugie, Y.; Miyazaki, T.; Miwa, M. Polyhedron 1986,5,1267. (f) Fox, M. I. Kinet. Catal. 1979,20, 416. A.; Cardona, R.; Gaillard, E. J. Am. Chem. SOC.1987, 109, 6347. (9) (7) Neumann, R.; &el, I. J. Chem. Soc., Chem. Commun. 1988,1285. Renneke, R. F.; Hill, C. L. New J. Chem. 1987,11,763. (h) Renneke, R. (8) (a) Matveev, K. I.; Kozhevnikov, I. V. Kinet. Catal. 1980,21,855. F.; Hill, C. L. J. Am. Chem. SOC.1988,110,5461. (i) Renneke, R. F.; Hill, (b) Matveev, K. I. Kinet. Catal. 1977,18,716. (c) Pachkovskaya, L. N.; C. L. Angew. Chem., Int. Ed. Engl. 1988,27, 1526. (j) Hiskia, A,; Pa- Matveev, K. I.; Il'inich, G. N.; Eremenko, N. K. Kinet. Catal. 1977, 18, paconstantinou, E. Polyhedron 1988, 7,477. 854.

0022-3263/89/1954-4607$01.50/0 0 1989 American Chemical Society 4608 J. Org. Chem., Vol. 54, No. 19, 1989 Neumann and Lissel

100 a 80

0 200 400 600 0 200 400 600 Time, min Time, min

6o 1

40120

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40 1 20

0 0 200 400 600 0 200 400 600 Time, mln Time, min Figure 1. Reaction profiles of the monoterpene dehydrogenation. Reaction conditions: 1mmol of monoterpene substrate, 0.25 mmol of tetraglyme, 0.01 mmol of H6PMoI0V2Om,2 mL of 1,2-dichloroethane, temperature = 70 "C, dioxygen or nitrogen pressure = 1 atm. Monoterpene substrate: (a) limonene (Oz),(b) limonene (Nz),(c) a-terpinene (02),(d) a-terpinene(Nz), (e) y-terpinene (OJ, (0 y-terpinene (N2). (A)Limonene; (0)a-terpinene; (0)y-terpinene; (0)terpinolene; (W) p-cymene. zation is preceeded by double-bond isomerization to an conditions with molecular oxygen in lieu of stoichiometric endocyclic position. reagents such as quinones (DDQ and p-~hloranil),~sele- nium, or sulfurlo or a high-temperature catalytic dehy- Results and Discussion drogenation with palladium or platinum." Further insight into the dehydrogenation reaction is The reaction system for the oxidative dehydrogenation gained by more carefully investigating the dehydrogenation reaction consists of the substrate, the H5PMo10V2040 of several monoterpene isomers such as limonene, 1, ter- catalyst dissolved in the presence of tetraglyme, CH30- pinolene, 2, a-terpinene, 3, and y-terpinene, 4, to p-cymene, (CH2CH20)4CH3,as complexing agent (see below), all in 5. The results of monoterpene dehydrogenation reaction 1,Zdichloroethane as solvent. Reactions are typically carried out at 70 "C under 1 atm of dioxygen. The results describing the aromatization of several substrates are given in Table I. Perusal of the results leads to the following observations. First, the cyclic dienes are dehydrogenated to the corresponding aromatic derivatives whereas the cyclic monoenes and saturated compounds are inert to dehydrogenation. Second, dioxygen is essential for the 1 2 3 4 5 catalytic cycle for there is no dehydrogenation (beyond the are given as reaction profiles in Figure 1. By examining stoichiometric amount of catalyst) in reactions carried out the profiles one may make the following observations. As under a nitrogen atmosphere. Third, the given catalyst described above in the absence of dioxygen there is no is unable to oxygenate any substrates under the given dehydrogenation although isomerization of the exocyclic conditions as no oxygenated products were found. This double bond occurs as can be seen in the isomerization of strongly indicates that dioxygen is not activated by the catalyst despite its being required for the reaction. Fourth, (9)Jackman, L. M.Adu. Org. Chem. 1960,2,329. conjugated dienes dehydrogenate more quickly than non- (10) (a) House, W. T.; Orchin, M. J. Am. Chem. Soc. 1960,82,639. (b) Silverwood, H. A.; Orchin, M. J. Org. Chem. 1962, 27, 3401. conjugated dienes. From a preparative synthetic point of (11)Rylander, P.N. Organic Syntheses with Noble Metal Catalysts; view this reaction enables a catalytic aromatization at mild Academic Press: New York, 1973. Aromatization of Hydrocarbons J. Org. Chem., Vol. 54, No. 19, 1989 4609 Scheme I catalyst and received similar results. These results differ from the results as given in Figure la and point to an electron-transfer mechanism, although acid-catalyzed isomerization cannot be ruled out. The dehydrogenation to the aromatic product is mostly through the more reactive a-terpinene (Figure IC).Sup- port for this pathway can be found by the virtual absence of a-terpinene in the dehydrogenation of limonene and y-terpinene (Figure la,e) despite its significant but slower formation in the absence of dioxygen (Figure lb,f). Therefore, in the dehydrogenation of limonene and y- terpinene the rate-determining step is the formation of a-terpinene, whereas in the a-terpinene dehydrogenation it appears the rate-determining step is the heteropoly acid reoxidation, otherwise one cannot explain the formation of 57% terpinolene in the latter case. Based on literature precedent%b dehydrogenation occurs by two (as shown by the UV-vis) successive electron transfers and proton abstractions from the endocyclic diene to the heteropoly acid forming the aromatic product and the reduced heteropoly limonene whereas isomerization of a- and y-terpinene with blue.16 The reduced heteropoly acid is reoxidized by two endocyclic double bonds is more limited. a-Terpinene dioxygen with formation of water. is more reactive than limonene or y-terpinene. This is At the beginning of our discussion we stated that the indicated both by the faster rate in Figure ICand by its heteropoly acid catalyst was dissolved into the 1,2-di- virtual absence in Figure 1, parts a and e. Another man- chloroethane solvent by complexation with tetraglyme. We ifestation of the greater reactivity of the a-terpinene is the wish to elaborate on this point and describe the isolation fact that its addition at room temperature to the catalytic of the tetraglyme-H5PMo10V2040complex. The com- solution immediately yields the reduced heteropoly blue plexation of heteropoly acids by crown ethers" has been with the simultaneous formation of p-cymene, whereas previously reported, so it is not surprising that an analo- limonene and y-terpinene reduce the catalyst at higher gous linear polyether will also ligate the heteropoly acid. (-50 "C)temperatures only. UV-vis spectra of 0.5 mM Thus, addition of tetraglyme to a mixture of H5PMolo- PV2M010040q-(q = 5, 6, or 7) dissolved in 1,2-dichloro- V20a and 172-dichloroethanesolubilizes the H5PMoloV2- ethane with 20 mM tetraglyme were taken after the ad- 040. Addition to the solution of an apolar hydrocarbon dition of the various terpenes. Addition of a-terpinene at such as hexane causes percipitation of the tetraglyme- room temperature yields a blue solution with an optical H5PMo,oV20,0complex. This complex as identified by density of 0.978 (c = 1956) at A,, 756 nm. On the other elementary analysis and 'H NMR spectroscopy (see Ex- hand addition of limonene or y-terpinene at 50 "C gives perimental Section) contains three tetraglyme ligands per a green solution with an optical density of 0.472 (E = 944) heteropoly acid and may be formulated as (tetra- at A,, 774 nm. Since the intensity of the intervalence glyme)3-H5PMo10V200 As for the 'H NMR spectra it is charge-transfer band is proportional to the number of informative to note that the peaks of the methylene electrons introduced and reduction beyond the one-elec- moieties were broadened (singlets were observed) and the tron stage causes a blue shift,I2 it is clear that in the case AA'-BB' coupling expected and observed in the 'H NMR of a-terpinene the heteropolyanion undergoes a two-elec- spectra of pure tetraglyme is lost. A probable explanation tron reduction whereas limonene or y-terpinene transfer is the interaction of the acidic protons from H~Mo10V20a one electron. with the oxygens atoms of the tetraglyme causing line A plausible reaction scheme that satisfactorally explains broadening. It may therefore, be concluded that the actual all the results above may be as represented in Scheme I. catalytic species is not the free H5PMo10V2040but the This scheme implies that the isomerization takes place complexed heteropoly acid (tetraglyme)3-H5PM~10V2040. through reversible electron transfer between the substrate Work is under way to crystallize this compound in order and heteropoly compound (perhaps through a charge- to characterize the molecular structure. transfer complex) as is indicated by the one-electron reduction of H5PMo10V2040by both limonene and y-ter- Conclusion pinene. Alternatively, the monoterpene isomerizations could be explained by the well-known acid catalysis.13 In A methodology has been developed for the liquid-phase fact, similar isomerizations of linear alkenes catalyzed by catalytic oxidative dehydrogenation at moderate temper- heteropoly acids have been recently reported.14 However, atures of cyclic dienes to the corresponding aromatic de- in the past acid-catalyzed isomerization of limonene has rivatives by use of (tetraglyme)3-H5PMoloV20adissolved been shown to yield in addition to polymeric tars di- in 1,2-dichloroethane in the presence of dioxygen. Aro- methylbicyclo[3.2.1]octenes,a-terpinene, p-menthenes,and matization is preceded by isomerization for dienes with p-cymene, the latter compounds being formed by inter- an exocyclic double bond. molecular proton transfer.15 We have repeated such a procedure using the conditions in the footnote of Table Experimental Section I with polyphosphoric acid instead of H5PMoloV20a as Materials and Instruments. All reagents and solvents used were obtained commercially and were used without further pu-

(12)Cf. ref la, p 109. ~ (13)Egleff, G.;Hulla, G.; Komerewsky, V. I. Isomerization of Pure (16) A reviewer also suggested the possibility of dehydrogenation oc- Hydrocarbons; Reinhold Publishing Corp.: New York, 1942. curring through hydrogen abstraction followed by rearrangements. (14)Nayak, V. S.; Moffat, J. B. Appl. Catal. 1988,36, 127. (17)Noguchi, H.; Yoshio, M.; Nakamura, H.; Nagamatsu, M. Rikoga- (15)Cf.: Accrombessy, G.;Blanchard, M.; Petit, F.; Germain, J. E. kuba shuho (Saga Daigaku) 1982, 10, 119; Chem. Abstr. 1982, 97, Bull. Chim. SOC.Fr. 1974, 705. 1923650. 4610 J. Org. Chem., Vol. 54, No. 19, 1989 Neumann and Lissel rification. The heteropoly acid H5PMo10V2010was prepared by of 530 pm and length 10 m. Reactions on a preparative scale were the common literature procedure.18 The catalyst is in fact a performed using 10 times the amounts quoted above. hydrated mixture of stereoisomers and was identifiedl9 by its 61V Dehydrogenation of the Monoterpenes: Reaction Profiles. NMR spectrum at 78.864 MHz with VOC13 as external standard The heteropoly catalyst H6PMoloVzOa (0.01 mmol, 20 mg) was on a Brucker WH-300 spectrometer. 8 ppm (integration) -527.9 dissolved in 2 mL of 1,2-dichloroethane by addition tetraglyme (l),-533.6 (4), -536.4 (4), and -543.2 (1). From the 51V NMR (0.25 mmol,55 pL) in a Wheaton reaction vial. The monoterpene data there is no evidence of impurities. GLC measurements were substrate (1 mmol, 136 mg) was added, and the vial was closed made with a HP-5890 equipped with FID detector and helium with an air-tight Teflon septum. The reaction mixture was then carrier gas. Peaks were quantified by a HP-3396A integrator after purged with dioxygen (or nitrogen where applicable) and kept calibration with known amounts of the compounds analysed. under 1 atm of pressure and heated to 70 OC. Reaction samples Elementary analysis were performed at the Hebrew University were taken at the various intervals and analyzed by GLC. The microanalytical center. Atomic absorbtion was performed using column used was a 10 m (length) X 530 pm (i.d.) cross-linked a GBC single-beam spectrometer. 'H NMR spectroscopy was FFAP at a column temperature of 50 "C. Retention times were performed on a Brucker WP-200-SY spectrometer. limonene 1.55 min, terpinolene 2.70 min, a-terpinene 1.30 min, Typical Procedure for the Oxidative Dehydrogenation y-terpinene 2.05 min, and p-cymene 2.40 min. Other isomers in Reactions. The heteropoly catalyst H6PMoloViOo(0.05 mmol, trace amounts were also detected. All peaks were base-line 100 mg) wiw dissolved in 10 mL of 1,2-dichloroethaneby addition separated. tetraglyme (1.0 mmol, 220 pL). After the solution was brought Preparation of the (Tetraglyme)s-H~MoloVzOoComplex. to the reaction temperature of 70 OC, the reaction substrate (5.0 The heteropoly H~MoloVzOo(0.025 mmol,50 mg)was dissolved mmol) was added and dioxygen was slowly bubbled through the in 5 mL of 1,2-dichloroethaneby addition of tetraglyme (1.0 "01, solution. At the end of the reaction the organic phase was ex- 220 pL). Hexane (5 mL) was added to the warmed solution (50 tracted with water to remove the catalyst and analysed by GLC. "C), and the mixture was slowly cooled. The precipitated complex The columns used were HP-5 (cross-linked 5% phenylmethyl was collected (yield was 40 mg, -65%) and analyzed by ele- silicone) or cross-linked FFAP. Both columns have a column i.d. mentary analysis and atomic absorption. The calculated (found) percentages for (tetraglpe)3-H&'MoloVzOIoare C, 14.98 (15.15); H, 3.06 (3.33); P, 1.29 (1.13); V, 4.23 (4.08); Mo, 39.87 (39.56). The (18) Tsigdinos, G. A.; Hallada, C. J. Znorg. Chem. 1968, 7, 437. 'H NMR spectra in CDSCOCDSat 200 MHz: 6 3.27 (3 H, s, (19) ODonnell, S. E.; Pope, M. T. J. Chem. SOC.,Dalton Trans. 1976, terminal methyl), 3.47 (2 H, broad s, terminal methylene), and 2290. 3.55 (6 H, s, internal methylene).